States hire private military or security companies [PMSCs/contractors] in armed conflict and occupation to fulfil tasks formerly exclusively handled by soldiers, including combat, guarding and protection, and detention and interrogation. PMSC personnel, like soldiers, can and do violate or act incompatibly with International Humanitarian Law and Human Rights Law. Relying on the International Law Commission's Articles on State Responsibility, the article compares the responsibility of states for such conduct of their soldiers with that which states incur with respect to the conduct of contractors they hire. It reveals a regulatory gap which states seeking to reduce their exposure to international responsibility can exploit. Positive obligations of states under International Humanitarian Law narrow this gap to some degree. An analysis of the duty to prevent demonstrates that the potential of positive Human Rights Law obligations to bridge the gap - although important - remains limited by their due diligence nature, and problems of extraterritorial applicability. It is then argued that the conduct of certain contractors exercising coercive functions can be attributed to the hiring state as that of 'persons forming part of its armed forces' in the sense of the customary provision enshrined in Article 3 of Hague Convention IV of 1907 and Article 91 of Additional Protocol I. Where this is the case, the state will be responsible for their conduct as it would be for that of its soldiers, which fully eliminates the regulatory gap.